Condensation of 2-alkoxypropenals with N,N- and N,O-1,2-binucleophiles. A route to 2-(1′-alkoxyvinyl)imidazolidines and -oxazolidines

Article abstract of DOI:10.1007/s10593-009-0213-y

Condensation of 2-ethoxypropenal with diaminoethylene in different solvents (CHCl₃, MeCN, H₂O, DMSO) at room temperature gives an equilibrium mixture (1:1-1.5) of tautomeric 2-(1′-ethoxyvinyl)-1,3- imidazolidine and 2-aminoethylimine of 2-ethoxypropenal as well as 1,2-bis(2′-ethoxypropenyl-ideneamino)ethylene. The latter is readily prepared in quantitative yield using a twofold excess of the aldehyde. ¹H NMR was used to demonstrate the effect of heating on the dynamics of the ring-chain tautomeric equilibrium. Reaction of the 2-alkoxypropenals with N-methyl- and N,N′-diphenyl-1,2-diaminoethylenes and with N-phenylaminoethanol gives only the corresponding substituted imidazolidines in 43-95% yield.

Full text of DOI:10.1007/s10593-009-0213-y

Chemistry of Heterocyclic Compounds, Vol. 44, No. 12, 2008
CONDENSATION OF 2-ALKOXYPROPENALS
WITH N,N- AND N,O-1,2-BINUCLEOPHILES.
A ROUTE TO 2-(1'-ALKOXYVINYL)IMIDAZO-
LIDINES AND -OXAZOLIDINES*
N. A. Keiko, N. V. Vchislo, L. G. Stepanova, L. I. Larina,
Yu. A. Chuvashev, and E. A. Funtikova
Condensation of 2-ethoxypropenal with diaminoethylene in different solvents (CHCl3, MeCN, H2O,
DMSO) at room temperature gives an equilibrium mixture (1:1-1.5) of tautomeric 2-(1'-ethoxyvinyl)-
1,3-imidazolidine and 2-aminoethylimine of 2-ethoxypropenal as well as 1,2-bis(2'-ethoxypropenyl-
ideneamino)ethylene. The latter is readily prepared in quantitative yield using a twofold excess of the
1
aldehyde. H NMR was used to demonstrate the effect of heating on the dynamics of the ring-chain
tautomeric equilibrium. Reaction of the 2-alkoxypropenals with N-methyl- and N,N'-diphenyl-
1,2-diaminoethylenes and with N-phenylaminoethanol gives only the corresponding substituted
imidazolidines in 43-95% yield.
Keywords: 2-alkoxypropenals, 1,2-diaminoethylenes, imidazolidines, oxazolidines, condensation,
tautomerism.
The reactions of α,β-unsaturated aldehydes with amines occur by various routes. It is known that the
reaction of secondary amines with a series of 2-halo-2-alkenals occurs via ipso substitution of the halogen atom
[1]. Along with this route there are possible 1,4-addition reactions with subsequent condensation to give 1,2-di-
aminoethanes and 1,3-bis(amino)-2-haloolefines [2]. Piperidine and morpholine add to 2-ethoxypropenal at
positions 1, 2 or 1, 4 with subsequent condensation of the product with another molecule of the amine. Mixtures
of the isomers of 3,3- or 1,3-diamino-2-ethoxy-1-propenes are formed in the ratio 1:6 in the first case or 3:2 in
the second [3]. Evidently the reaction course depends markedly on the nature of the amine.
The reaction of N-acetyl-2-aminopropenal with N,N'-disubstituted diaminoethylenes followed by
hydrogenation with NaBH4 forms 1,4-hexahydrodiazepines [4].
Primary amines form Schiff bases with 2-ethoxypropenal [5, 6] although it is noted that in the reaction
of acrolein with nucleosides the amino groups of cytosine and thymine can add at the 1,4 positions [7]. β-Amino
thiols and β-amino alcohols form a mixture of tautomeric thiazolidines and imino thiols [8] or oxazolidines and
imino alcohols [9] with 2-alkoxypropenals.
_______
* Dedicated to Academician of the Russian Academy of Sciences B. A. Trofimov in his 70th jubilee.
__________________________________________________________________________________________
A. E. Favorsky Irkutsk Institute of Chemistry, Russian Academy of Sciences, Irkutsk 664033, e-mail:
keiko@irioch.irk.ru. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1809-1815,
December, 2008. Original article submitted May 29, 2008.
1466
0009-3122/08/4412-1466©2008 Springer Science+Business Media, Inc.

Thus the direction of attack on the C=C bond or the CHO group of the α,β-unsaturated aldehyde
depends markedly on the electronic effect of the substituent in the substrate and in the nucleophilic reagent as
well as the reaction conditions.
In continuing our work on the chemistry of α-functionally substituted α,β-unsaturated aldehydes and,
especially the mutual electronic and steric effect of the captodative substituents in a gem position to the double
bond we have now studied the regioselectivity of the addition to 2-alkoxypropenals 1 of bifunctional nitrogen
nucleophiles 2, i.e. ethylenediamine, its N- and N,N'-substituted derivatives and also of 2-(N-phenyl-
amino)ethanol.
The realization of a possible synthesis of novel 1,3-imidazolidines based on the indicated reaction is
connected with the marked interest in the chemistry and the biochemical reactions of this class of compounds at
this time [10-17].
We have shown that the reaction of 2-ethoxypropenal (1a) with an equimolar amount of 1,2-ethylene-
diamine (2a) occurs exothermically in the absence of a solvent. Full conversion of aldehyde 1 is seen after
several minutes to give three compounds: 3-(2'-aminoethylimino)-2-ethoxypropene (3a), 2-(1'-ethoxyvinyl)-
imidazolidine (4a), and 1,2-bis(2'-ethoxypropylideneamino)ethane (5) in the ratio 1:2:1.
OEt
OEt
H2N
O
N
+
+
NH2
NH2
1a
2a
3a
OEt
OEt
OEt
H
N
N
+
+
N
N
H
4a
5
This process occurs more slowly in solvents (CHCl3, CH2Cl2, MeCN, H2O). 100% Conversion of
aldehyde 1a occurs over 1 h at 24ºC. It should be noted that the isomeric compounds 3a and 4a are found in an
equilibrium ratio (1:1-1.5) in all cases and the starting diamine 2a always remains partially unused. The content
of the diimine 5 is 7-17% in the reaction mixture. When the mixture is stored (20ºC) this amount increases
somewhat. It is evidently related to the ready reversibility of the condensation of aldehydes with primary
amines. Enough traces of moisture in the reaction mixture (or in the air) lead to hydrolysis of compound 4a. The
propenal 1a so formed then attacks the free amino group of imine 3a resulting in the thermodynamically stable
diimine 5. The water formed in the latter condensation again hydrolyzes compound 4a etc. We have, in fact,
shown that reaction of diamine 2a with a twofold excess of aldehyde 1a gives only the diimine 5 in 100% yield
(1H NMR).
It was not possible to form principally one of the primary condensation products 3a or 4a when carrying
out the reaction in CHCl3, even with a 2-3-fold excess of diamine 2a. However, the use of a more polar solvent
(DMSO) gave a 3a:4a ratio of 1:1.7 (without combining with water of condensation).
Since the ring-chain tautomerism of 5-6-membered 1,3-heterocycles affects their reactivity (and hence
determines their synthetic use) and is also important for understanding and predicting biochemical processes
[10, 16, 17] we have tried to show that compounds 3a and 4a are not isomeric (one of which is a labile
intermediate) but are actually tautomers. With this in mind we have studied the change in their ratio upon
heating the reaction mixture in CDCl3 solution in the NMR spectrometer ampule. After mixing an equimolar
amount of reagents over 1.5 h at room temperature a steady (equilibrium) mixture of 3a:4a (0.9:1) is observed
(three control measurements). Heating the ampule for 1 min to 60ºC changes this ratio to 1.38:1 and after 5 min
at 60ºC to 1.5:1. Cooling of the ampule to 27ºC (for 10 min) caused the 3a:4a ratio to return to the starting
1467

equilibrium value (0.9:1). Hence the rate of mutual thermal conversion of compounds 3a, 4a amounts to minutes
and is readily monitored by NMR. Meanwhile, examples are known in which the ratio of tautomers is reached
over 3.7 h for oxazolidine–imino alcohols [18] or 7.5 h for thiazolidine-iminothiols [19].
The fraction of diimine 5 does not change upon heating at 60ºC for 6 min in the NMR ampule. Even at
higher temperature (80ºC, 1 h, acetonitrile) the reaction of reagents 1a and 2a with subsequent cooling leads to
the standard ratio 3a:4a 0.7:1 with an 8% content of diimine 5 in the mixture. However, it was not possible to
separate the individual compounds 3a and 4a. Upon distillation (block temperature 136ºC) the main reaction
product (60-80%) becomes the diimine 5 which was separated in up to 52% yield. Compound 5 is the only
reaction product seen when the reaction mixture was analyzed by GLC-MS.
N-Methylated diamines almost always show a higher tendency to cyclization than their analogs
unsubstituted on the nitrogen atom [20]. In fact, such a dependence was shown in the reaction of 2-ethoxy-
propenal 1a with N-methyldiaminoethylene (2b) (20ºC, 1 h) giving a 100% yield (1H NMR) of 2-(1'-ethoxyvinyl)-
1-methylimidazolidine (4b). Since the starting diamine 2b has a strongly basic secondary amino group an initial
attack in two directions (a and b) might be expected.
OR
OR
OR
X
R2
R2
N
XR1
R1 X
O
NHR2
+
N
–H2O
OEt
OH
1a,b
2a–d
1
R
4a–f
OEt
N
a
NHMe
H
N
1a + 2b
3b
NHMe
–H2O
b
4a
OH
1 a R = Et, b R = Me; 2 a X = NH, R1 = R2 = H, b X = NH, R1 = H, R2 = Me,
c X = NH, R1 = R2 = Ph, d X = O, R1 = H, R2 = Ph; 4 a X = N, R = Et, R1 = R2 = H,
b X = N, R = Et, R1 = H, R2 = Me, c X = N, R = Et, R1 = R2 = Ph, d X = N, R = Me,
R1 = R2 = Ph, e X = O, R = Et, R1 −, R2 = Ph, f X = О, R = Me, R1 −, R2 = Ph
However, we never observed the linear isomer 3b at reasonable concentration although it can be readily
identified from the CH=N group proton signal (7.5 ppm for aliphatic systems) [21].
As a result of the lowering of the basicity of the nitrogen atoms via the acceptor effects of the phenyl
groups in N,N'-diphenyl-1,2-diaminoethane (2c) the reaction with 2-ethoxy- and 2-methoxypropenals 1a and 1b
is much slower than for the diaminoethylenes 2a and 2b. At 24ºC the reaction in CHCl3 only occurs to the extent
of 30% after 1 h and 80% after 24 h. The 2-(1'-alkoxyvinyl)-1,3-diphenyl-1,3-imidazolidines 4c and 4d
respectively are stable, crystalline substances.
The reaction of the N-phenylaminoethanol (2d) with the 2-alkoxypropenals 1a and 1b occurs just slowly
and in the same direction. According to 1H NMR data the yield of 2-[1'-ethoxyvinyl]-3-phenyl-oxazolidine (4e)
is 40% after 1 h and 92% after 24 h. Microwave irradiation markedly speeds up this reaction. Thus, the targeted
product 4e yield is 80% after 4 min, i.e. the rate of the reaction increases by 30 times and the rate of formation
of compound 4f increases by 24 times.
The marked lowering of the basicity of the nitrogen atoms due to the electronegative effects of the
substituents in N,N'-diacetyl-N,N'-diaminoethylene results in this diamine not taking part in a reaction with the
2-ethoxypropenal 1a under comparable conditions (20ºC, 1-25 h).
1468

Hence we have discussed the influence of the electron effects of several substituents at the reactive
centers of nitrogen binucleophiles on their regioselectivity and reaction rate with 2-alkoxypropenals. In the
conditions chosen the existence of ring-chain tautomerism for the novel 1,3-imidazolidine 4a containing a
vinyloxy group at position 2 has been shown. This fact confirms and develops the recent publication dependence
showing that only those N-unsubstituted 1,3-imidazolidines in which the position 2 has a 2-iso-propyl group or
two alkyl groups take part in the ring-chain equilibrium while imidazolidines with linear alkyl substituents in
position 2 (Me, Et, Pr) exist in CDCl3 exclusively as the cyclic tautomers. For the 2-phenyl- substituted
imidazolidine the tautomeric equilibrium is fully shifted to the open isomer side [10]. The introduction of an
alkoxyvinyl group at position 2 of the imidazolidines opens up the possibility of their further functionalization.
EXPERIMENTAL
1H and 13C NMR spectra were recorded on a Bruker DPX-400 spectrometer (400 and 100 MHz
respectively) using CDCl3 with HMDS as internal standard. IR spectra were obtained on a Specord IR-75
spectrometer. Chromato-mass spectrometric analysis was performed on a Hewlett-Packard 5971A instrument
(EI, 70 eV, mass selective detector), HP-5890 chromatograph, Ultra-2 column (5% phenylmethylsilicone),
evaporator temperature 250ºC, column thermostat temperature 70-280ºC, rate of temperature increase 20
deg./min. An LG MS-1904H 700 watt microwave oven was used for the microwave irradiation with a C-20.1
pyrometer (technoa, from -18ºC to +500ºC).
1,2-Bis(2'-ethoxypropylideneamino)ethane (5). The diamine 2a (1.12 g, 18.7 mmol) was added to a
solution of the propenal 1a (1.87 g, 18.87 mmol) in CHCl3 (2 ml). After several minutes the 1H NMR spectrum
of the reaction mixture showed 3-(2'-aminoethylimino)-2-ethoxypropene (3a) and 2-(1'-ethoxyvinyl)imida-
zolidine (4a), ratio 3a:4a = 1:1.3. 1H NMR spectrum of compound 3a, δ, ppm (J, Hz): 1.21 (3H, t, J = 6.9, CH3);
3.00 (2H, m, CH2); 3.56 (2H, m, CH2); 3.88 (2H, q, J = 6.9, OCH2); 4.54 (1H, d, J = 2.0, =CH2); 4.57 (1H, d,
1
J = 2.0, =CH2); 7.58 (1H, s, N=CH). H NMR spectrum of compound 4a, δ, ppm (J, Hz): 1.32 (3H, t, J = 7.0,
CH3); 2.87 (2H, dd, J = 10.2, J = 11.5, CH2 ring); 3.04 (2H, dd, J = 10.2, J = 11.5, CH2 ring); 3.77 (2H, q,
J = 7.0, OCH2); 3.99 (1H, d, J = 2.0, =CH2); 4.25 (1H, d, J = 2.0, =CH2); 4.18 (1H, s, CH). The reaction mixture
was dried over MgSO4 and solvent was removed at reduced pressure. Distillation in vacuo of the residue gave
1
the product 5 (2.19 g, 52.3%) with bp 127ºC (1 mm Hg). H NMR spectrum of compound 5, δ, ppm (J, Hz):
1.39 (6H, t, J = 7.0, CH3); 3.80 (4H, s, CH2); 3.87 (4H, q, J = 7.0, 2OCH2); 4.50 (2H, d, J = 2.0, =CH2); 4.56
13
(2H, d, J = 2.0, =CH2); 7.61 (1H, s, N=CH). C NMR spectrum, δ, ppm: 14.46 (CH3); 61.17 (NCH2); 63.59
(OCH2); 93.99 (=CH2); 157.76 (=CH–); 159.78 (–CH=). GLC-MS, m/z (Irel, %): 223 [M-1]+ (1), 195 (15), 181
(13), 127 (32), 98 (100), 84 (56), 68 (14), 55 (38), 41 (22), 29 (20). Found, %: C 63.92; H 8.92; N 12.39.
C12H20N2O2. Calculated, %: C 64.28; H 8.93; N 12.5.
2-(1'-Ethoxyvinyl)-1-methyl-1,3-imidazolidine (4b). The propenal 1a (0.95 g, 9.5 mmol) was added to
a solution of the N-methyldiaminoethylene 2b (0.714 g, 9.5 mmol) in CHCl3 (2 ml). The reaction mixture was
held for 1 h at 24ºC and the solvent was evaporated in vacuo to give compound 4b (0.604 g, 43%) with bp 45ºC
(1 mm Hg). IR spectrum (thin film), ν, cm-1: 1610 and 1650 (C=C), 3100 (sharp), 3320 (broad) (NH). 1H NMR
spectrum, δ, ppm (J, Hz): 1.33 (3H, t, J = 7.0, CH3); 2.25 (1H, m, CH2 ring); 2.30 (3H, s, NCH3); 2.98 (1H, m,
CH2); 3.19 (2H, m, CH2); 3.30 (1H, s, NH); 3.79 (2H, two q, J = 7.0, OCH2); 4.07 (H, d, J = 2.0, =CH2); 4.18
(1H, d, J = 2.0, =CH2). GLC-MS, m/z (Irel, %): 156 [M]+ (10), 127 (6), 113 (5), 98 (3), 85 [M-CH2=COEt]+
(100), 84 [M-CH2=COEt-H]+ (24), 68 (2), 56 (5), 44 (31), 28 (5). Found, %: C 61.31; H 10.22; N 17.88.
C8H16N2O. Calculated, %: C 61.54; H 10.26; N 17.95.
2-(1'-Ethoxyvinyl)-1,3-diphenyl-1,3-imidazolidine (4c). Propenal 1a (1.08 g, 10.8 mmol) was added
to a solution of the diamine 2c (2.3 g, 10.8 mmol) in EtOH (3 ml). The reaction mixture was held for about
1469

16 h at room temperature. The crystals produced were filtered off and recrystallized twice from ethanol to give
1
compound 4c (2.74 g, 86.32%) with mp 68.2ºC. H NMR spectrum, δ, ppm (J, Hz): 1.11 (3H, t, J = 7.0, CH3);
3.53 (2H, q, J = 7.0, OCH2); 3.67 (2H, m, CH2); 3.73 (2H, m, CH2); 4.13 (1H, d, J = 2.0, =CH2); 4.3 (1H, d,
J = 2.0, =CH2); 5.45 (1H, s, CH); 6.72 (6H, m, o-, p-H Ph); 7.18 (4H, m, m-H Ph). 13C NMR spectrum, δ, ppm:
14.41 (CH3); 45.79 ((CH2)2); 62.83 (OCH2); 75.83 (CH); 86.11 (=CH2); 113.39 (Cm); 117.46 (Cp); 128.77 (Co);
144.81 (Cipso); 159.95 (=C–O). Found, %: C 77.73; H 7.62; N 9.47. C19H22N2O. Calculated, %: C 77.51; H 7.53;
N 9.52.
2-(1'-Methoxyvinyl)-1,3-diphenyl-1,3-imidazolidine (4d). Propenal 1b (1.25 g, 6 mmol) was added to
a solution of diamine 2c (0.5 g, 6 mmol) in CHCl3 (2 ml). The reaction mixture was held for about 16 h at room
temperature. The crystals produced were filtered and recrystallized twice from CHCl3 to give compound 4d
1
(1.61 g, 95.8%) with mp 94.8ºC. H NMR spectrum, δ, ppm (J, Hz): 3.37 (3H, s, OCH3); 3.66 (2H, m, CH2);
3.74 (2H, m, CH2); 4.16 (1H, d, J = 2.6, =CH2); 4.33 (1H, d, J = 2.6, =CH2); 5.45 (1H, d, CH); 6.71 (6H, m,
13
o-, p-H Ph); 7.18 (4H, m, m-H Ph). C NMR spectrum, δ, ppm: 45.26 (CH2); 4.25 (CH3); 75.32 (CH); 85.33
((CH2)2); 112.81 (Cm); 117.01 (Cp); 128.32 (Co); 144.81 (arom Cipso); 159.95 (=C–O). Found, %: C 77.34;
H 7.22; N 10.02. C18H20N2O. Calculated, %: C 77.14; H 7.14; N 10.0.
2-(1'-Ethoxyvinyl)-3-phenyl-1,3-oxazolidine (4e). Propenal 1a (1.5 g, 15 mmol) was added to a
solution of N-phenyl-2-aminoethanol (2d) (2.05 g, 15 mmol) in CHCl3 (2 ml). The reaction mixture was held at
24ºC for 24 h. Solvent was evaporated under reduced pressure and the residue was distilled in vacuo to give
compound 4e (1.65 g, 66.1%) with bp 120-125ºC (1 mm Hg). With the use of microwave irradiation (1 min×4,
420 watts, 48.6ºC) the yield of the product 4e was 80%. 1H NMR spectrum, δ, ppm (J, Hz): 1.32 (3H, t, J = 7.0,
CH3); 3.42 and 3.54 (2H, two m, OCH2); 3.76 (2H, m, CH2N); 4.09 and 4.29 (2H, two d, J = 2.0, =CH2); 4.18
(2H, m, CH2O ring); 5.21 (1H, s, CH); 6.55 (2H, d, J = 8.0, o-H Ph); 6.72 (1H, t, J = 7.6, p-H Ph); 7.15 (2H, dd,
J = 8.0, J = 7.6, m-H Ph). 13C NMR spectrum, δ, ppm: 14.11 (CH3); 46.64 (NCH2); 63.02 (OCH2); 65.10 (OCH2
ring); 83.93 (=CH2); 89.74 (CH); 112.48 (Cm); 117.30 (Cp); 128.64 (Co); 145.11 (Cipso); 159.04 (=C–O). Found,
%: C 71.42; H 7.90; N 6.41. C13H17NO2. Calculated, %: C 71.23; H 7.76; N 6.39.
3-(1'-Methoxyvinyl)-3-phenyl-1,3-oxazolidine (4f). Propenal 1b (1.217 g, 14 mmol) was added to a
solution of aminoethanol 2c (1.937 g, 14 mmol) in CHCl3 (2 ml). The reaction mixture was held for 24 h at
room temperature. Solvent was evaporated under reduced pressure and the residue was distilled in vacuo to give
compound 4f (1.794 g, 62.5%) with bp 137ºC (1 mm Hg). Carrying out the reaction in a microwave oven
(1 min × 5, 420 watts, 47.6ºC) increased the reaction rate 24 times. 1H NMR spectrum, δ, ppm (J, Hz): 3.57 (2H,
m, CH2N); 3.61 (3H, s, OCH3); 4.18 (3H, m, CH2O and 1H from =CH2); 4.34 (1H, d, J = 2.3, =CH2); 5.26 (1H,
s, CH); 6.59 (2H, d, J = 7.9, o-H Ph); 6.70 (1H, t, J = 7.3, p-H Ph); 7.16 (2H, dd, J = 7.3, J = 7.9, m-H Ph). 13C
NMR spectrum, δ, ppm: 47.29 (NCH2); 55.65 (OCH3); 65.78 (OCH2); 84.40 (=CH2); 90.22 (CH); 113.07 (Cm);
117.92 (Cp); 129.30 (Co); 145.76 (Cipso); 160.46 (=C–O). Found, %: C 69.96; H 7.42; N 7.05. C12H15NO2.
Calculated, %: C 70.24; H 7.32; N 6.83.
This work was carried out with the financial support of the Russian fund for Basic Research (grant
No. 08-03-00396).
REFERENCES
1.
2.
3.
N. A. Keiko, A. Y. Rulev, I. D. Kalikhman, and M. G. Voronkov, Synthesis, 446 (1998).
A. Yu. Rulev, N. A. Keiko, and M. G. Voronkov, Izv. Akad. Nauk, Ser. Khim., 135 (1996).
N. A. Keiko, A. Yu. Rulev, I. D. Kalikhman, and M. G. Voronkov, Izv. Akad. Nauk SSSR, Ser. Khim.,
2610 (1985).
4.
H. Harada, T. Morie, Y. Hirokawa, and S. Kato, Chem. Pharm. Bull., 44, 2205 (1996).
1470

5.
6.
7.
N. A. Keiko, A. P. Chichkarev, and M. G. Voronkov, Izv. Akad Nauk SSSR, Ser. Khim., 579 (1973).
J. Barluenga, M. Tomas, L. A. Lopez, and A. Suarez-Sobrino, Synthesis, 697 (1997).
R. Shapiro, R. S. Sodum, D. W. Everett, and S. K. Kundu, IARC Sci. Publ., 70, 165 (1986); Chem.
Abstr., 106, 190370 (1987).
8.
9.`
N. A. Keiko, E. A. Funtikova, L. G. Stepanova, and L. I. Larina, Zh. Org. Khim., 41, 529 (2005).
N. A. Keiko, E. A. Funtikova, L. G. Stepanova, Yu. A. Chuvashev, and L. I. Larina, Zh. Org. Khim., 39,
1546 (2003).
10.
L. Lázár and F. Fülöp, Eur. J. Org. Chem., 3025 (2003).
11.
K. N. Zelenin, V. V. Alekseev, K. Pikhlaiya, and V. V. Ovcharenko, Izv. Akad. Nauk, Ser. Khim., 197
(2002).
12.
13.
14.
15.
16.
17.
18.
19.
K. Tanaka and R. Shiraishi, Green Chem., 2, 272 (2000).
M. Ishihara and H. Togo, Synlett, 227 (2006).
A. P. Blum, T. Ritter, and R. H. Grubbs, Organometallics, 26, 2122 (2007).
S. Kagabu, R. Ishihara, J. Hieda, K. Nishimura, and Y. Naruse, Agricult. Food Chem., 55, 812 (2007).
J. B. Lambert, G. Wang, D. E. Husel, and L. C. Takiff, J. Org. Chem., 52, 68 (1987).
J. B. Lambert and M. W. Majchrzak, J. Am. Chem. Soc., 102, 3588 (1980).
S. L. Spassov, L. Markova, O. Argirov, and Tz. Obretenov, J. Mol. Struct., 147, 105 (1986).
N. A. Keiko, E. A. Funtikova, L. G. Stepanova, Yu. A. Chuvashev, L. I. Larina, and M. G. Voronkov,
J. Sulfur Chem., 25, 351 (2004).
20.
21.
R. E. Val'ter, Usp. Khim., 51, 1374 (1982).
J. J. Pesek and J. H. Frost, Tetrahedron, 31, 907 (1975).
1471